Over fifty years ago it was recognized that alkylbenzene sulfonates (ABS) were quite effective detergents superior to natural soaps in many respects. Because of their lower price, their price stability, and their effectiveness in a wide range of detergent formulations, ABS rapidly displaced soaps in household laundry and dishwashing applications and became the standard surfactants for the detergent industry.
The alkylbenzene sulfonates as initially prepared had substantial branching in the alkyl chain. This situation was maintained until the early 1960's when it became apparent that the branched alkyl-based detergents were contributing to the pollution of lakes and streams and forming relatively stable foams. Examination of the problem showed that the branched structure of the alkyl chains was not susceptible to rapid biodegradation and the surfactant properties of the detergent thus persisted for long periods of time. This was not the case earlier when natural soaps were used, because of the rapid biodegradation of the linear chains in natural soaps.
After recognizing the biodegradability of ABS based on alkylation by linear olefins, industry turned its attention to the production of these unbranched olefins and their subsequent use in the production of linear alkyl benzenes. Processes were developed for efficient alkylation of benzene by available feedstocks containing linear olefins, and the production of linear alkyl benzenes (LAB) became another reliable process broadly available to the petroleum and petrochemical industry. It gradually evolved that HF-catalyzed alkylation was particularly effective in LAB production, and an HF-based alkylation process became the industry standard. More recently solid acid catalysts have undergone intensive development as an alternative to liquid HF.
As desirable as solid catalysts may be as an alternative to liquid HF, a continuing impediment to their development has been their short lifetime. Although all catalysts lose some portion of their activity with continued use, the solid catalysts used to date in aromatic alkylation tend to deactivate rather quickly. Although their deactivation can be retarded by increasing alkylation reaction temperature, raising the reaction temperature tends to decrease product linearity, which is an undesirable outcome. Conversely, lowering the reaction temperature increases product linearity, an exceedingly desirable result, but exacerbates catalyst deactivation leading to useful catalyst lifetimes on the order of only several hours. Thus, it is clear that solid catalysts can be best used in the continuous alkylation of aromatics only where effective, convenient, and inexpensive means of catalyst regeneration are available.
Solid catalysts used for the alkylation of aromatic compounds by olefins, especially those in the 6-20 carbon atom range, usually are deactivated by byproducts which are preferentially adsorbed by the catalysts. Such byproducts include, for example, polynuclear hydrocarbons in the 10-20 carbon atom range formed in the dehydration of C6-C20 linear paraffins and also include products of higher molecular weight than the desired monoalkyl benzenes, e.g., di- and trialkyl benzenes, as well as olefin oligomers. Although it can be readily appreciated that such catalyst deactivating agents or "poisons" are an unavoidable adjunct of aromatic alkylation, hence catalyst deactivation also is unavoidable, fortunately it has been observed that the deactivating agents can be readily desorbed from the catalyst by washing the catalyst with the aromatic reactant. Thus, catalyst reactivation, or catalyst regeneration as the term is more commonly employed, is conveniently effected by flushing the catalyst with aromatic reactants to remove accumulated poisons from the catalyst surface, generally with restoration of 100% of catalyst activity. It would be particularly advantageous to integrate a continuous alkylation process with a method of removing catalyst deactivating agents virtually as formed, thereby preventing catalyst deactivation and obviating a separate catalyst regeneration stage. Our invention accomplishes this latter objective.
U.S. Pat. No. 4,028,430 describes a simulated moving bed reaction process which integrates catalyst regeneration and alkylation, as does U.S. Pat. No. 4,008,291 and 4,072,729. Description of only the '430 patent will suffice to illustrate the general teachings. The invention uses a fixed bed of solid catalyst containing four zones arranged in series with a fluid flow path connecting each adjacent zone as well as connecting the fourth zone to the first zone. The solid catalyst is one which deactivates due to adsorption onto the catalyst of higher molecular weight byproducts, but the solid catalyst can be readily reactivated by washing it with a suitable solvent to desorb these higher molecular weight deactivating materials.
The simulated moving bed reaction process as described operates in the following way. Reactants are introduced to the top of zone 1 and reaction products are withdrawn at the boundary of zones 1 and 2. A solvent known to effectively desorb from the catalyst those materials responsible for catalyst reactivation is introduced at the top of zone 3. In many cases the solvent is one of the reactants normally used in excess. Solvent flows from the top of zone 3 toward the bottom and a stream containing the desorbed material responsible for catalyst deactivation is withdrawn at the junction of zones 3 and 4. Since all or nearly all of the catalyst poisons are removed before zone 4, the catalyst in zone 4 is a reactivated or regenerated catalyst.
In practice the foregoing invention is performed using application of a simulated moving bed technique. In this technique the catalyst bed actually consists of a series of catalyst sub-beds, each with a fluid flow path connecting adjacent sub-beds, and means for shifting the points of inlet and outlet streams of the process. Thus the various distinct zones move spatially along the catalyst bed in a sort of circular fashion.
The foregoing process certainly has advantages which make it commercially the most preferred and profitable process for many reactions, nonetheless it is attended both by a high capital outlay and a relatively high operational cost. To reduce the process cost even further we have devised a process which accomplishes the same result as continuous alkylation with continuous catalyst regeneration using only a single catalyst zone where the input and output streams remain stationary relative to the catalyst bed. This obviates the need for construction of a relatively complex reactor, thus reducing capital costs, and the operational expense associated with our technique is more comparable to that of a conventional fixed bed reactor rather than a simulated moving bed reactor.
Our invention uses a single fixed bed catalyst zone where both alkylation and removal of catalyst deactivating agents occur. Reactants flow into the bed at or near one terminus of the catalyst zone along with desorbent. Reaction occurs along the catalyst bed and reaction products are collected at the second terminus of the catalyst zone. Our invention is characterized by a continuous flow of desorbent and a pulse flow of reactants. In a variant the desorbent flow is interrupted during at least a portion of the reactant flow pulse. In this way the materials causing catalyst deactivation are periodically collected at the second terminus of the catalyst bed, alternating with collection of products, and are thus removed from the catalyst bed with continuous maintenance of catalytic activity. Our process is simple, quite effective, and affords alkylated aromatic products of high quality.
Our process is distinguished from the prior art in two important ways. First is what may be termed a philosophical difference; whereas the prior art focuses attention on catalyst regeneration after deactivation, ours emphasizes prevention of deactivation. The second difference is a functional one; whereas the prior art may use a 2-cycle process (a reaction and a regeneration cycle) each of a relatively long duration, in our invention each cycle time is short, as is their sum or the periodicity of our process. Ancillary to this is the characteristic that the process of our invention requires only a single catalyst zone because of the short periodicity, whereas the prior art in practice requires at least two catalyst zones because the regeneration time (when no product is formed) is long.
The process which is our invention is characterized by two cycles: a reaction cycle, when both olefin and aromatic flow into the catalyst zone to effect alkylation, and a flush cycle when only a desorbent flows into the catalyst zone. For simplicity of exposition only we consider the case where the desorbent is also the aromatic being alkylated. In our process the periodicity is short, on the order of minutes, whereas the practice of the prior art utilizes a long periodicity measured in terms of several hours or even days. Periodicity can be expressed not only as time but equivalently in terms of fractional catalyst deactivation or fractional catalyst carbon buildup, and however the periodicity is expressed it is quite short--shorter by at least an order of magnitude--relative to the prior art practices of aromatic alkylation. Thus, a reaction cycle of about 10 minutes and a flush cycle of 10 minutes, to afford an overall periodicity of 20 minutes, is representative of our process and is dearly substantially different from prior art practice. A second distinguishing feature, easy to be overlooked at first glance, is that our process uses only a single catalyst bed in which both the reaction and regeneration cycles are effected sequentially and repeatedly. Thus, e.g., the prior art swing bed process utilizes at least two catalyst beds, one in which reaction is being effected and another in which regeneration is being effected concurrently. This obviously contrasts starkly with our invention, where both reaction and flushing are effected in the same catalyst bed sequentially (not concurrently) over a short time period, with the reaction-flushing cycle repeated essentially continuously until the catalyst bed is no longer usable. The foregoing also should make it clear that the relative times for the reaction and flush cycles will depend upon many factors, such as olefins used, the aromatic alkylated, reaction temperatures, aromatic to olefin ratio, and so on. But it should be equally dear that the periodicity and relative times of the reaction and flush cycles are readily within the purview of one skilled in the art and necessitate only a modicum of experimentation. Thus, optimization of the process of our invention requires only routine experimentation which the skilled worker will recognize.
However important and effective may be our process in the alkylation of aromatics to detergent alkylates, it is capable of broad generalization. Thus, although the principles elaborated below will be first specified with regard to detergent alkylation for the purpose of clarity and exemplification, it will be appreciated by the skilled worker that the principles are general and applicable to many reaction processes. It is to be clearly understood that our invention is not a narrow one, with the examples of its use increasing as the appreciation of its applicability increases.